The production of polyurethane integral skin foams is known and is described for example, in German Auslegeschrift No. 1,196,864.
These integral skin foams are produced, for example, by charging a foamable reactive mixture based on compounds having isocyanate-reactive hydrogen atoms and polyisocyanates into a closed mold. Water and/or fluorine hydrocarbons are used as blowing agents according to the prior art. Catalysts of the type known for the production of polyurethane foams are generally also used.
By selecting suitable starting components, in particular by selecting the molecular weight and the functionality of these components, it is possible to produce flexible, as well as rigid foams and intermediate variations. The compact outer skin is achieved in this process by introducing a larger quantity of a foamable mixture into the mold than required to fill the mold cavity by free foaming. The internal wall of the mold thus generally causes cooling of the reaction mixture and condensation of the preferred organic blowing agent so that the blowing reaction ceases on the internal wall of the mold and the compact outer skin is formed.
Suitable organic blowing agents for this process include commercially-available fluorinated and/or halogenated hydrocarbons since they have sufficiently low boiling points and they do not form explosive gaseous mixtures when mixed with air. Fluorotrichloromethane and/or methylene chloride, in particular, are among the suitable blowing agents for producing polyurethane integral skin foams. Pentane or similar non-halogenated hydrocarbons are inadvisable blowing agents as expensive safety precautions are needed due to the low explosion limit of pentane-air mixtures.
Objections to both fluorotrichloromethane and methylene chloride have been expressed recently for ecological reasons, however.
It is therefore desirable to develop alternative blowing agents for the production of polyurethane foams, particularly for polyurethane integral skin foams.
As already mentioned, water may be used as a blowing agent in the polyurethane system. While free-rise polyurethane foams of excellent quality may be produced by means of this procedure, integral skin foams cannot be so produced as the surface texture, as well as the integral structure, of the foam deteriorates in comparison with molded integral skin foams using fluorine hydrocarbons as blowing agents. Another disadvantage resides in the fact that the water has to be added to the reactive mixture as an individual component immediately prior to foaming since at least partial saponification of the indispensable tin compounds (for example dibutyl-(IV)-dilaurate) occurs during the addition of suitable quantities of water to the polyol component, generally already containing the foam catalyst. This is manifested in an uncontrolled drop in activity in the already activated polyol component.
Other alternative blowing agents include compounds which decompose at temperatures above room temperature and thus give off a blowing gas. Examples include azodicarbonamide, azo-bis-isobutyronitrile or diphenylene oxide disulphohydrazine and the pyrocarbonic esters described in German Offenlegungsschrift No. 2,524,834 (U.S. Pat. No. 4,070,310) and the benzoxazines according to German Auslegeschrift No. 2,218,328 which gave off CO.sub.2. In order to use them as blowing agents, these compounds must have a relatively low decomposition temperature which, according to general experience, should lie well below 100.degree. C., as the blowing agents must be effective at the beginning of the urethanization reaction. Compounds having such a low decomposition temperature, however, are obviously sensitive during storage and they require careful handling of a type which cannot be guaranteed commercially in many cases by the processors of polyurethane foams. Moreover, it is frequently characteristic of these compounds that uncontrolled decomposition might occur during storage so that they also represent a safety risk.
The use of alkane aldoximes as blowing agents is mentioned in German Patent No. 1,112,285 (British Pat. No. 908,337). The aldoximes react with NCO groups and give off CO.sub.2. However, the corresponding alkyl nitrile is formed simultaneously and, in the case of the examples mentioned therein, in which acetonitrile, butyronitrile or isobutyronitrile are formed when acetaldoxime, butyraldoxime or isobutyaldoxime are used as blowing agents. These compounds are all physiologically harmful and have low flash points.